sf: Move SPI flash drivers to defconfig

There are already Kconfig options for SPI flash drivers, but we
have not moved them from config.h to defconfig files. This commit
does this in a batch.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/T4240RDB_defconfig b/configs/T4240RDB_defconfig
index efb0389..217471a 100644
--- a/configs/T4240RDB_defconfig
+++ b/configs/T4240RDB_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_T4240RDB=y
 CONFIG_SYS_EXTRA_OPTIONS="PPC_T4240"
 CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_SST=y
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y